The victims were working in a field in the As-Sawah village around 80 kilometers (50 miles) northwest of the city of Baalbek when they picked up a strange looking object. The object exploded, killing Mahmoud al-Arab, 24, and wounding four others, including a 13-year old girl.
A witness, Khaled Hussein, said al-Arab was instantly killed while some of the others were seriously wounded.
Police were investigating the incident. Security officials said the blast was possibly caused by an Israeli cluster bomb from the war last summer. The officials spoke on condition of anonymity in line with military regulations.
Israel has been accused of firing as many as 4 million cluster bombs into Lebanon during the war. United Nations demining experts say up to 1 million cluster bombs failed to explode immediately and continue to threaten civilians. Mines from the conflict have also killed civilians since the fighting ended.
A stronghold of the militant group Hezbollah, Baalbek and surrounding areas were repeatedly hit by Israeli airstrikes during the monthlong conflict.